Small angle neutron and X-ray scattering by poly(methyl methacrylate) chains

Abstract

The intensity of radiation scattering by poly(methyl methacrylate) (PMMA) chains is computed as a function of scattering angle over the range 0 < μ = ( 4π λ) sin ( υ 2 ) < 0.3 A ̊ −1 , on the basis of a realistic rotational isomeric state model. The scattering functions F x ( μ), corresponding to Iμ 2, are developed for chains of x units in terms of the even moments 〈 r 2 p ij 〉 of the separation distance between pairs of the monomer units i and j. Whereas the theoretical scattering function F x ( μ) for isotactic PMMA increases monotonically with μ, for predominantly syndiotactic PMMA it exhibits a maximum at μ ≈ 0.05 A ̊ −1 . This is in agreement with experimental results on small angle neutron and X-ray scattering by PMMA (in bulk and in solution, respectively). The appearance of the maximum in F x ( μ), heretofore considered anomalous, is shown to be a direct consequence of the preference of racemic diads of PMMA for the trans, trans conformation and of the inequality of the skeletal bond angles at CH 2 and at the doubly substituted C α.